WASHINGTON (AP) – More than a third of U.S. states have some form of marijuana decriminalization, but only the District of Columbia’s pot policy is being singled out for a Congressional hearing.

Mayor Vincent Gray signed a bill in March that decriminalizes possession of small amounts of pot. It won’t take effect until after a review by Congress that is ongoing. Congress would be extremely unlikely to pass a resolution invalidating the law. But it could block the measure in other ways.

Today’s hearing will be the third in a series that a House Oversight subcommittee has held examining the conflict between state-level marijuana policies and federal law.

The committee is taking a close look at the district because of the abundance of federal property and federal law enforcement agencies in the nation’s capital.